GENERAL
All field devices shall be robust in construction, suitable for use in environmental conditions of 0-50 degrees Celsius and 10% – 95% relative humidity. The devices shall operate on 0-20VDC or 4-20mA signals and shall be adjustable for calibration. Set points shall be adjustable from 0-100% of the input range. Analogue devices shall provide long-term accuracy without need for regular calibration. A binder test point shall be installed in the pipework adjacent to each control device to provide for manual testing of pressure and/or temperature.
TEMPERATURE SENSORS
.1 General
Temperature sensors shall be factory calibrated and shall not require compensation for cable lengths, etc. Accuracy of sensors shall be +/- 0.3 degree C.
The following working range shall apply.
Location of Sensors Range
Room 15 to 40 deg. C
Duct 5 to 40 deg. C
Outdoor Air 10 to 40 deg. C
Chilled Water 3 to 20 deg. C
Condenser Water 10 – 40 deg C
.2 Duct Mounted Sensors
- Shall consist of a transmitter and a remote sensor.
- The transmitter section of the transducer shall be mounted on the outside of ducts clear of insulation and sheathing and have a tamper proof cover.
- The sensor shall have a separate mounting flange with snap on connection to permit sensor removal for site calibration and service, and shall be protected from physical damage, dust and
moisture. It shall have a minimum insertion length of 200 mm, with temperature averaging along its entire length. The sensor shall be mounted in the duct in a position that senses the
true average temperature of the air in the duct.
.3 Conditioner Mounted Sensors
- Shall consist of a transmitter and a remote averaging sensor.
- The transmitter section of the transducer shall be mounted on the outside of conditioner housing clear of insulation and sheathing and have a tamper proof cover.
- The averaging sensor shall be mounted in the conditioner housing in a position that senses the true average temperature of the air in the conditioner. The sensor shall have a separate
mounting flange with snap on connection to permit sensor removal for site calibration and service, and shall be protected from physical damage, dust and moisture.
.4 Immersion Type Sensors
- For pipe or tank insertion. Shall be sufficient length to ensure accurate measurement and shall be installed in wells designed to permit easy removal for inspection or repair.
- Wells shall be of 100 mm stainless steel and be designed to suit the sensor so that response time remains suitable for the application. The sensor shall be in thermal contact with the
immersed surface of the well. Wells for insulated pipework shall have sufficient length to ensure that sensor is not hindered by the insulation and wells shall be mounted such that
condensation does not run into the sensor head. - The transmitter section of the transducer shall be mounted adjacent to the well in which the sensor is mounted. It shall be possible to remove the transmitter and/or the sensor without
removing the well from the pipe.
CARBON MONOXIDE SENSORS
Carbon monoxide sensors shall provide a calibrated output signal with a minimum measuring range of 0-200 ppm CO.
DIFFERENTIAL PRESSURE SWITCHES
Pressure differential switches shall have SPDT change-over contacts, switching at an adjustable differential pressure setpoint and rated for the medium temperature being utilised.
FLOW SWITCHES
Flow switches shall be of the paddle type equipped with SPDT contacts to establish proof of flow. Flow switches shall be of the vapour-proof type. Switches shall be mounted in accessible
locations where the effects of turbulence etc. will not cause unsatisfactory operation, and shall be installed in accordance with manufacturer’s recommendations.
POSITION SWITCHES
Position switches shall be capable of being displayed in percent open notation.
MOTORISED DAMPER ACTUATORS
- Actuators shall be linear or rotary type as required for the application.
- Actuators shall be spring return failsafe as specified under MECHANICAL SERVICES CONTROLS – FUNCTIONAL DESCRIPTION, GENERAL. Where damper failsafe condition is
not specified, it shall be failsafe open. - The input signal to the actuator shall be 0-20 VDC or 4-20 mA as required. The output shaft of the actuator shall be pushed/rotated between the limits of its travel directly proportional to the
input signal. The actuator shall be fitted with a manual operator such that, in the event of a power failure or other failure, manual operation can be achieved without disconnecting damper
linkage. - Actuators shall include spring return to failsafe position.
- All dampers shall be failsafe open unless otherwise stated.
- Actuators shall include limit end switches to prevent damage to the electric actuators. Continuous feedback shall be employed to position the motor shaft relative to input signal.
- Actuator gearboxes shall have heavy duty gears.
- Linear actuators shall provide a linear (push-pull) thrust without the use of cams, gears or linkages, and shall not require maintenance or readjustment.
- Actuators shall be electro-servo or electrohydraulic type and shall have a minimum number of moving parts.
- Actuators shall incorporate an additional input for remote override or minimum positioning. Actuators shall be provided with all necessary mounting brackets, push rods, etc.
- Motorised dampers shall incorporate position switches as indicated in the MCS POINTS SCHEDULE.
CONTROL VALVES
- Control valves shall be of the modified linear or equal percentage type and be suitable for the temperatures and pressures of the particular services in which they are installed. Valve bodies
shall be of bronze or cast iron with stainless steel trim and renewable discs and seats. - Control valves 50 mm and below shall be threaded connection. Valves 65 mm and above shall be flanged. Valves shall be available with a body pressure rating of greater than 16 bar and
rangeability of better than 100:1. - Threaded valves shall be of gunmetal (bronze) and flanged valves of cast iron or cast steel. Threaded valves shall be provided with pipe coupling pieces.
- All valves whether two port or three port shall be closed when the spindle is in the up position.
- Two port valves shall have an equal percentage control characteristic.
- Three port valves shall have an equal percentage characteristic on the through port and a linear characteristic on the bypass port. Three port valves shall be piped for mixing service.
- Valve actuators shall be mounted directly on the control valve without the need for separate linkages and the need for any adjustment of the actuator stroke. Actuators shall have a manual
operation capability. - Weatherproof covers shall be placed over all valves and actuators that are exposed to the weather or immediately inside external louvres.
- Two way control valves shall be sized for a pressure drop at design flow as follows:
- Not less than 130% of pressure drop through the coil being controlled.
- Not less than 15% of the worst case pressure differential across the closed valve. For pressure controlled circuits, this shall mean the worst case pressure differential at the circuit pressure control point. For non-pressure controlled circuits, this shall mean pump shut off head.
- Not greater than 60 kPa, provided that valves in ceiling spaces or other areas where noise has to be limited shall be sized for a pressure drop not greater than 35 kPa.
- The above specifications are anticipated to require Secondary CHW control valves to be selected within the range of 25-60 kPa.
- Two way control valves shall be capable of closing against the worst case pressure differential that will occur across the closed valve. For pressure controlled circuits, this shall mean the worst case pressure differential at the circuit pressure control point. For non-pressure controlled circuits, this shall mean pump shut off head.
- Valves shall provide tight shut-off against this head.
- The above specification is anticipated to require Secondary CHW control valves to have a close-off rating of 200 kPa.
- The Controls Specialist shall be responsible for the proper selection of control valves, including obtaining system operating and pressure characteristics from the Sub-Contractor. He shall
make good at no extra charge any misapplication of control valves.